The aspects of the disclosed embodiments belong to the field of transport aircraft such as civilian aircraft or military transport aircraft.
More particularly, the aspects of the disclosed embodiments relate to an aircraft architecture that provides improved precise control of the longitudinal balance of the aircraft.
For reasons of performance, stability, and control, an aircraft must respect certain constraints between the relative positions along the longitudinal axis, the center of gravity of the aircraft, and characteristic points of the aircraft aerodynamics.
When balanced in level flight, not only does the center of aerodynamic thrust have to correspond to the center of gravity of the aircraft, but the position of the different aerodynamic foci relative to the balance in pitch has to be situated both rather close to the center of gravity and in general behind this center of gravity.
Behind it for reasons of natural stability and to guarantee the possibility of piloting the aircraft in case of the failure of any system for increasing longitudinal stability, and close enough for reasons of performance while avoiding large setting angles of the horizontal stabilizers, a source of aerodynamic drag, to ensure the longitudinal balance of the aircraft.
These phenomena are perfectly known to the aircraft designers who nevertheless have to take into account the fact that the position of the center of gravity is variable for a given aircraft, in particular because of the loading of the aircraft, which cannot always be totally controlled, for example the number and placement of the passengers for civilian passenger transport aircraft, and during a mission because of the fuel consumption during the flight as well as the displacements of the aerodynamic foci depending on the Mach number of the flight from compressibility phenomena.
To respond to these requirements, a method, historically the oldest, consists of defining the aircraft, its geometry, and the allowed positions of the center of gravity or allowed region of centering, so that the aircraft has acceptable flight characteristics under all conditions that it may encounter in flight.
In this type of aircraft, the longitudinal balance of the aircraft around the pitch axis is ensured by the horizontal stabilizer whose setting angles permit the application of aerodynamic lift forces upward or downward to compensate for the torque resulting from the shifting of the center of thrust from the lift of the wing relative to the center of gravity, which are both developments in the longitudinal direction of the aircraft that depend on the mission and the flight path.
To take advantage of the performance of modern systems, some aircraft also make use of fuel transfer during flight between the principal reservoirs in the wings and auxiliary reservoirs in the rear section of the aircraft distant from the wings, in particular in the rear horizontal stabilizer, to displace the position of the center of gravity of the aircraft.
This method of managing the center of gravity is relatively effective because the auxiliary reservoirs of the rear fins are distant from the center of gravity and benefit from a substantial lever arm.
However, this solution has the drawback of making the fuel system more complex, and makes it necessary to make the structure of the fin tight to serve as a reservoir, and to add pipes and transfer pumps.
This solution is also limited by the volume that can be contained by the rear fins, or by an additional fuselage reservoir, as the case may be.
Furthermore, when the fuel has to be consumed and it no longer permits providing the function of controlled displacement of the center of gravity, it is accordingly necessary to consider the aircraft in the situation in which the fuel ensemble will be consumed, which limits the capabilities of such a system relative to what would theoretically be possible, and in particular it is not possible in practice to reduce the dimensions of the fins even if the position of the center of gravity relative to the aerodynamic foci were perfectly controlled.